It is known that the centrifugation of blood can separate the latter into three distinct layers, namely a least-dense layer constituted by the blood plasma and forming the upper layer, an intermediate layer consisting predominantly of leukocytes and thrombocytes, known as the buffy-coat layer and of intermediate density, and a lower layer constituted predominantly of erythrocytes or in the form of a red blood cell concentrate having the greatest density.
Separation of these layers is frequently desired so that, for example, the plasma can be stored separately from the red cell concentrate, the red cell concentrate can be processed into valuable blood products, and/or the buffy-coat layer can be processed for a similar purpose
While the centrifugation of blood has been done in rigid vessels or in centrifuges which separate the components in a continuous manner, in recent years the collection of blood in flexible bags has dominated and so too has the whole blood been increasingly centrifuged in such bags which have flexible tubes opening into the upper ends of the bags.
In order to decant the plasma and buffy-coat layers from such bags, press arrangements have been provided which can have vertical press plates between which the bag can be introduced and one of each is advanced toward the other to drive the upper layer of plasma out of the tube followed by the buffy-coat layer.
Such a press generally made use of a movable press plate which was somewhat smaller in size than the stationary press plate and which covered substantially the area of the red cell concentrate so that by squeezing the latter inwardly, the layers above the red cell concentrate can be effectively driven out of the tube.
A press of this type is described in U.S. Pat. No. 4,350,585 and has experienced a problem at least with respect to the expulsion of the buffy-coat layer from the blood collection bag. In this arrangement, after the plasma is ejected by squeezing the red cell concentrate with the relatively small movable press plate, all or part of the buffy-coat layer tends to remain in the bag above this movable press plate and must be expelled from the bag manually or by other manipulations applied to the portion of the bag above the red cell concentrate. In practically all cases some mixing of the low-density buffy-coat layer with the red cell concentrate occurs and this is undesirable.
The successive removal of blood plasma and buffy coat from a blood bag has also been described in Vox Sanguinis 39 (1980), 46-59, see especially pages 48-51.
In this system after the blood plasma has been driven off above the buffy coat, a clamp is applied to the bag between the buffy coat and the red blood cell concentrate to allow the separation. A stomach-operation clamp is used for this purpose.
The quality of the separation here as well is dependent upon the manipulative skills of the individual.